1. Field of the Invention
The present invention relates to a coordinate measuring machine.
2. Description of Related Art
A known coordinate measuring machine includes: a probe for measuring an object; a movement mechanism for moving the probe; and a controller for controlling the movement mechanism (see, for instance, Document 1: JP-T-2009-524032). The coordinate measuring machine is designed to stop the movement of the probe when the probe or the movement mechanism and the object collide with each other, for instance, on account of an operation error of a user while the probe is moved by the movement mechanism.
When the probe or the movement mechanism and the object collide with each other, a load applied on the movement mechanism increases, resulting in an increase in a current value for moving the probe by the movement mechanism. Thus, in order to judge the collision between the probe or the movement mechanism and the object, the coordinate measuring machine may be designed to judge the occurrence of the collision between the probe or the movement mechanism and the object when the current value exceeds a predetermined threshold.
However, when an acceleration of the movement of the probe by the movement mechanism is small (i.e. low speed), the current value is small. On the other hand, when the acceleration is large (i.e. high speed), the current value is large. Accordingly, with a constant threshold for judging the collision between the probe or the movement mechanism and the object, when the speed for moving the probe by the movement mechanism is slow, it takes nonnegligible time for the coordinate measuring machine to judge that the probe or the movement mechanism and the object collide with each other after the collision actually occurred.
The coordinate measuring machine disclosed in Document 1 detects a current value flowing to an electric motor for driving a movement mechanism including a column, horizontal support, horizontal feeder and coordinate measuring device, where the detected current value is compared with a stepwise threshold that is set in accordance with a target speed and acceleration for moving a probe head (probe) by the movement mechanism to judge the occurrence of the collision between the probe or the movement mechanism and the object. According to the above arrangement, it is possible for the coordinate measuring machine to speedily judge the occurrence of collision between the probe or the movement mechanism and the object as compared to an arrangement in which a constant threshold is used.
However, since the coordinate measuring machine disclosed in Document 1 sets the threshold in a stepwise manner in accordance with the target speed and the acceleration, an appropriate threshold cannot be set for the target speed and acceleration.